1. Field of the Invention
The present invention relates to semiconductor fabrication and more particularly to a lithography exposure apparatus (aligner) for transferring a circuit pattern from a mask or a reticle onto a sensitive substrate.
The present invention also relates to a system for detecting a focal point on a workpiece (wafer, substrate or plate etc.) and for detecting a tilt of the workpiece, which is applicable to certain kinds of apparatus such as an apparatus for manufacturing a workpiece or imaging a desired pattern in a surface of a workpiece using a laser or electron beam and an apparatus for optically inspecting the state of a surface of a workpiece.
2. Description of the Related Art
Recently, dynamic random access memory semiconductor chips (DRAMs) having an integration density of 64 Mbits have been mass-produced by semiconductor fabrication techniques. Such chips are manufactured by exposing a semiconductor wafer to images of circuit patterns to form e.g. ten or more layers of circuit patterns in a superposition manner.
Presently, lithography apparatuses used for such chip fabrication are projection aligners in which a circuit pattern drawn in a chromium layer on a reticle (mask plate) is transferred onto a resist layer on a wafer surface through a 1/4 or 1/5 reduction optical imaging system by irradiating the reticle with i-line radiation (wavelength: 365 nm) of a mercury discharge lamp or pulse light having a wavelength of 248 nm from a KrF excimer laser.
Projection exposure apparatuses (projection aligners) used for this purpose are generally grouped, according to the types of imaging optical system, into those using a step-and-repeat system, i.e., so-called steppers, and those using a step-and-scan system which has attracted attention in recent years.
In the step-and-repeat system, a process is repeated in which, each time a wafer is moved to a certain extent in a stepping manner, a pattern image on a reticle is projected on a part of the wafer by using a reduction projection lens system formed only of a refractive optical material (lens element) and having a circular image field or an unit magnification projection lens system formed of a refractive optical material (lens element), a prism mirror and a concave mirror and having a noncircular image field to expose a shot area on the wafer or plate to the pattern image.
In the step-and-scan system, a wafer is exposed to an image of a portion of a circuit pattern on a reticle (for example, in the form of a circular-arc slit) which is projected on the wafer through a projection optical system. Simultaneously, the reticle and the wafer are continuously moved at constant speeds at a speed ratio according to the projection magnification, thus exposing one shot area on the wafer to the image of the entire circuit pattern on the reticle in a scanning manner.
For example, as described on pp 256 to 269 of SPIE Vol. 922 Optical/Laser Microlithography (1988), the step-and-scan system is arranged so that, after one shot area on the wafer has been scanned and exposed, the wafer is moved one step for exposure of an adjacent shot area, and so that the effective image field of the projection optical system is limited to a circular-arc slit. Also, the projection optical system is considered to be a combination of a plurality of refractive optical elements and a plurality of reflecting optical elements, such as one disclosed in U.S. Pat. No. 4,747,678 (to Shafer).
U.S. Pat. No. 5,194,839 (to Nishi) discloses an example of an aligner in which a step-and-scan system is realized by mounting a stepper reduction projection lens having a circular image field. This publication also discloses a method in which a pattern image projected at the time of scanning exposure is transferred onto a wafer by increasing the depth of focus (DOF) by a predetermined amount on the wafer.
In the field of lithography technology, it is now desirable to be able to fabricate semiconductor memory chips having an integration density and fineness of the 1 or 4 Gbit class by light exposure. Since light exposure techniques have a long technological history and are based on a large amount of accumulated know-how, it is convenient to continue use of light exposure techniques. It is also advantageous to use light exposure techniques considering drawbacks of alternative electron beam exposure or X-ray technologies.
It is believed that resolutions in terms of minimum line width (feature width) of about 0.18 .mu.m and 0.13 .mu.m are required with respect to 1 Gbit and 4 Gbit memory chips, respectively. To achieve resolution of such a line width, far ultraviolet rays having a wavelength of 200 nm or shorter, e.g., those produced by an ArF excimer laser, are used for illumination for irradiating the reticle pattern.
As optical vitreous materials having a suitable transmittance with respect to far ultraviolet rays (having a wave-length of 400 nm or shorter), quartz (S.sub.i O.sub.2), fluorite CaF.sub.s, lithium fluoride (L.sub.i F.sub.2), magnesium fluoride (MgF.sub.2) and so on are generally known. Quartz and fluorite are optical vitreous materials indispensable for forming a projection optical system having high resolution in the range of far ultraviolet rays.
However, it is necessary to consider the fact that, if the numerical aperture (NA) of a projection optical system is increased to attain high resolution while the field size is increased, the diameter of lens elements made of quartz or fluorite becomes so large that it is difficult to manufacture such lens elements.
Also, if the numerical aperture (NA) of the projection optical system is increased, the depth of focus (DOF) .DELTA.F is inevitably reduced. In general, the depth of focus .DELTA.F is defined by wavelength, numerical aperture NA, a process coefficient Kf (0&lt;Kf&lt;1) as shown below if the Rayleigh's theory of imaging formation is applied: EQU .DELTA.F=Kf.multidot.(.lambda./NA.sup.2)
Accordingly, the depth of focus .DELTA.F in the atmosphere (air) is about 0.240 .mu.m if the wavelength is 193 nm, that is, equal to that of ArF excimer laser light, the numerical aperture NA is set to about 0.75 and the process coefficient Kf is 0.7. In this case, the theoretical resolution (minimum line width) .DELTA.R is expressed by the following equation using process coefficient Kr (0&lt;Kf&lt;1): EQU .DELTA.R=Kr.multidot.(.lambda./NA)
Accordingly, under the above-mentioned conditions, the resolution .DELTA.R is about 0.154 .mu.m if the process coefficient Kr is 0.6.
As described above, while it is necessary to increase the numerical aperture of the projection optical system in order to improve the resolution, it is important to notice that the depth of focus decreases abruptly if the numerical aperture is increased. If the depth of focus is small, there is a need to improve the accuracy, reproducibility and stability with which an automatic focusing system for coincidence between the best imaging plane of the projection optical system and the resist layer surface on the wafer is controlled.
On the other hand, considering the projection optical system from the standpoint of design and manufacturing, a configuration is possible in which the numerical aperture is increased without increasing the field size. However, if the numerical aperture is set to a substantially larger value, the diameter of lens elements is so large that it is difficult to form and work the optical vitreous material (e.g. quartz and fluorite).
Then, as a means for improving the resolution without largely increasing the numerical aperture of the projection optical system, an immersion projection method may be used in which the space between the wafer and the projection optical system is filled with a liquid, see U.S. Pat. No. 4,346,164 (to Tabarelli).
In this immersion projection method, the air space between the wafer and the optical element constituting the projection optical system on the projection end side (image plane side) is filled with a liquid having a refractive index close to the refractive index of the photoresist layer, to increase the effective numerical aperture of the projection optical system seen from the wafer side, i.e. improving the resolution. This immersion projection method is expected to attain good imaging performance by selecting the liquid used.
Projection aligners as presently known generally are provided with an automatic focusing (AF) system for precisely controlling the relative positions of the wafer and the projection optical system so that the wafer surface coincides with the optimum imaging plane (reticle conjugate plane) of the projection optical system. This AF system includes a surface position detection sensor for detecting a change in the height position (Z-direction position) of the wafer surface in a non-contact manner, and a Z-adjustment mechanism for adjusting the spacing between the projection optical system and the wafer on the basis of the detected change.
Also in projection aligners presently used an optical type or air micrometer type sensor is used as the surface position detection sensor, and a holder (and a Z-stage) for supporting the wafer, provided as the Z-adjustment mechanism, is moved vertically to sub-micron accuracy.
If such an AF system is provided in an aligner to which the immersion projection method is applied, it is natural that an air micrometer type sensor cannot be used and an optical sensor is exclusively used since the wafer is held in a liquid. In such a case, an optical focus sensor, such as one disclosed in U.S. Pat. No. 4,650,983 (to Suwa), for example, is constructed so that a measuring beam (an imaging beam of a slit image) is obliquely projected into the projection field on the wafer and so that the beam reflected by the wafer surface is received by a photoelectric detector through a light receiving slit. The change in the height position of the wafer surface, i.e., the amount of focus error, is detected from a change in the position of the reflected beam occurring at the light receiving slit.
If an oblique incident light type focus sensor such as the one disclosed in U.S. Pat. No. 4,650,983 is directly mounted in a projection aligner in which the conventional projection optical system having a working distance of 10 to 20 mm is immersed in a liquid, a problem described below arises. In such a case, it is necessary to set in the liquid the optical system of the projected beam emitted from a projecting objective lens of the focus sensor to reach the projection field of the projection optical system on the wafer and the optical system of the reflected beam reflected by the wafer to reach a light receiving objective lens.
Therefore, the beam of the focus sensor travels through a long distance in the liquid, so that unless the temperature distribution in the liquid is stabilized with high accuracy, the projected beam and the received beam fluctuate by a change in refractive index due to a temperature nonuniformity, resulting in deterioration in the accuracy of focus detection (detection of the height position of the wafer surface).
Moreover, to achieve a resolution of 0.15 .lambda.m or less by the immersion projection method, it is necessary to set the working distance of the projection optical system to a sufficiently small value, as mentioned above. Therefore, oblique projection itself of the projected beam of the oblique incident light type focus sensor from the space between the projection optical system and the wafer toward the projection area on the wafer becomes difficult to perform. For this reason, one important question arises as to how an automatic focusing system applicable to the immersion projection method is arranged.
On the other hand, aligners (exposure apparatus) having an unit magnification type (hereinafter described as "1.times.") projection optical systems are being used in the field of manufacturing liquid crystal display devices (flat panel displays) as well as in the field of manufacturing semiconductor devices. Recently, for this kind of aligner, a system has been proposed in which a plurality of 1.times. projection optical systems of a certain type are arranged and in which a mask and a photosensitive plate are moved integrally with each other for scanning. It is desirable that, ideally, the working distance of the 1.times. projection optical systems used is extremely small. Each 1.times. projection optical system is of a single Dyson type such as that disclosed in U.S. Pat. No. 4,391,494 (to Hershel) or a double Dyson type such as that disclosed in U.S. Pat. No. 5,298,939 (to Swanson et al.).
In an aligner having such a Dyson type projection optical system, the working distance (spacing between the exit surface of a prism mirror and the image plane) can be sufficiently reduced to limit various aberrations or distortions of the projected image to such small values that there is practically no problem due to the aberrations or distortions. In this kind of aligner, therefore, a detection area on the photosensitive substrate of focus detection by the focus sensor (e.g., the irradiation position of the projected beam in the oblique incident light system or the air-exhaust position in the air micrometer system) is ordinarily set to a position deviating from the effective projection field region of the projection optical system, that is, set in an off-axis manner.
For this reason, it is impossible to actually detect whether the area of the substrate exposed to projected light from a circuit pattern is precisely adjusted in a best focus state or condition.
Also in apparatuses for writing a pattern on a substrate or to perform processing (or manufacturing) by using a spot of a laser beam or an electron beam, it is possible that the working distance between the substrate and the objective lens system (or an electronic lens system) for projecting the beam becomes so small that an AF sensor capable of detecting a focusing error of the processing position or the drawing position on the substrate surface in the field of the objective optical system cannot be mounted.
In such a case, the detection point of the AF sensor is only placed outside the field of the objective lens system to detect a focusing error, and it does not detect whether a focusing error occurs actually at the processing position or writing position in the field of the objective lens system.
The same can also be said with respect to an apparatus for optically inspecting a pattern drawn on a reticle or mask for photolithography or a fine pattern formed on a wafer. That is, this is because this kind of inspection apparatus is also provided with an objective lens system for inspection and because the end of the objective lens system faces a surface of an specimen (a plate) to be inspected while being spaced apart from same by a predetermined working distance.
Thus, if an objective lens system having a comparatively large magnifying power and high resolution is used, the working distance is so small that the same problem relating to the disposition of the AF sensor is encountered.